Driving has been on the decline in the United States since 2004, as researchers have documented every which way. What they still don't know, though, is precisely why. The answer likely has to do with some messy mix of rising gas prices, changing demographics, new technology, a souring economy and the shifting preferences of Millennial drivers. But it's tempting to lean on some of those explanations more heavily than others.

The economic theory is a particularly deceptive one. If you believe that driving trends have gone south because the economy has, too, then that means U.S. policy doesn't need to adjust to a new transportation reality less focused on cars. Wait long enough, and everything will go back to the old normal.

Years will have to pass before we can look back on this moment and know for sure if the decline in driving was primarily a product of the economy or something else. For now, though, the U.S. PIRG Education Fund has proposed a "natural experiment" with the data we do have: vehicle mileage per capita by state. If the economy is a major factor here, then states hardest hit by the recession—with the steepest rise in unemployment—would experience the most significant drop in driving, right? For one thing, unemployed people without jobs to drive to don't drive as much.

Or maybe not. This is clearly a simplistic experiment, but in a new report released today, U.S. PIRG could find no clear relationship at the state level between the health of the local economy and the size if its decline in driving.

To help understand this, keep in mind that states differ widely in how much people drive over the course of a year. In 2011, according to Federal Highway Administration data, national vehicle miles traveled per capita were 9,455 miles. At the bottom end of the spectrum, that figure was just 5,774 miles a year in the District of Columbia. In sparsely populated Wyoming, VMT per capita was more than 16,000 miles a year.

Since VMT peaked nationwide in 2004, driving has also declined in nearly every state:

But factors such as the rise of telecommuting or the increase of urbanization can't explain the differing rates of change between them. When author Phineas Baxandall then looked at the rates of state-by-state driving decreases relative to unemployment (or the change in unemployment) between 2005 and 2011, no great pattern emerged either. For the scatterplot-minded, this is what that non-relationship looks like:

Of the 23 states where driving declined faster than the national average, only six had unemployment rates that rose faster than the country as a whole. And most of the states with above-average increases in unemployment had below-average declines in driving.

In an experiment like this, it's impossible to isolate the role of the economy when states vary on so many other factors as well. But this picture does suggest, as Baxandall puts it, that the decline in driving is "about more than an economic aftershock."

Map images via "Moving Off the Road A State-by-State Analysis of the National Decline in Driving," by U.S. PIRG Education Fund.

This article is from the archive of our partner The Wire.

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